[PATCH] D33104: [BitVector] Implement find_[first/last]_[set/unset]_in

[Zachary Turner via Phabricator via llvm-commits]

zturner created this revision.

This effort started when I wanted a method that did the following:

  Given a BitVector and indices N and M, find the largest index less than M but greater than or equal to N which contains a set bit.

At first I just used `find_prev(M)` which I had added in an earlier patch, but I quickly discovered the performance of this was poor.  My `BitVector` had over 500,000 bits in it, and in the worst case this could cause an O(n) scan all the way to the beginning.  But I already had a lower bound on the range I wanted to search in, so I figured I could just make a function `find_last_in(LowerBound, PriorTo)`.

But it quickly became apparent that if we already have this function, then `find_last()` is just `find_last_in(0, Size)` and `find_prev(PriorTo)` is just `find_last_in(0, PriorTo)`.

In fact, all of the methods are generalizable this way.  For example, `find_first` and `find_next` reduce to `find_first_in(0, Size)` and `find_first_in(After, Size)`.  The generalizations to the `find_xxx_unset` family of methods is obvious.

So I went all the way.  There is now a `find_in` method for all combinations of `[first, last] x [set, unset]`, and all `find` methods delegate to these methods.


https://reviews.llvm.org/D33104

Files:
  llvm/include/llvm/ADT/BitVector.h
  llvm/unittests/ADT/BitVectorTest.cpp

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